In a blender, combine the pear, soy sauce, sake, sugar, garlic, 1/4 tsp. salt, and 3/4 tsp. pepper and process until smooth. Transfer to a medium bowl and stir in the scallions, sesame oil, and sesame seeds. Set aside 1/2 cup of the marinade in a small bowl for serving.
With the tip of a paring knife, poke the steak all over, and then slice it crosswise into 3 or 4 even pieces. Submerge the meat in the remaining marinade and let sit at room temperature while preparing the grill.
Prepare a grill for direct grilling over medium-high heat (about 400°F). Grill the steak, flipping once, until browned on both sides and medium rare, 6 to 7 minutes total. Let the steak rest for about 5 minutes, then thinly slice on the diagonal. Arrange the steak on a large platter with the rice and lettuce leaves for wrapping. Serve the reserved marinade on the side for drizzling.